FIG. 26 shows the configuration of a conventional typical optical head apparatus. The optical head apparatus is provided with a semiconductor laser 1, a collimator lens 2, a polarizing beam splitter 3, a quarter-wave plate 4, an objective lens 6, a cylindrical lens 8, a convex lens 9 and a photo-detector 10. The output light emitted by the semiconductor laser 1, which serves as a light source, is collimated by the collimator lens 2. The collimated light enters into the polarizing beam splitter 3 as a P-polarized light, and almost 100% thereof enters the quarter-wave plate 4 after passing through the polarizing beam splitter 3. The quarter-wave plate 4 converts the incoming light from linear to circular polarized light when the incoming light passes therethrough. The circular polarized light is focused by the objective lens 6 onto a disc 7, which is a sort of optical recording medium. The reflected light reflected by the disc 7 passes through the objective lens 6 in the opposite direction and enters the quarter-wave plate 4. The quarter-wave plate 4 converts the incoming light from circular to linear polarized light when the incoming light passes therethrough. The direction of polarization of the linear-polarized light on this return path is orthogonal to that of the linear-polarized light on the outward path. The linear-polarized light enters the polarizing beam splitter 3 as an S-polarized light, and almost 100% thereof enters the cylindrical lens 8, after being reflected. This light passes through the cylindrical lens 8 and the convex lens 9 and is received by the photo-detector 10.
Such an optical system, which incorporates a polarizing beam splitter and a quarter-wave plate, is referred to as a polarization optical system. The optical head apparatus using a polarization optical system is featured in that there is nearly no light loss in the polarizing beam splitter which separates the light of the outward path and the light of the return path for both of the return and outward paths. Therefore, the optical head apparatus using a polarization optical system offers a high optical output in recording operations, while offering a high S/N ratio in reproducing operations; the optical head apparatus using a polarization optical system is mainly used as an optical head apparatus adapted to write-once read-many optical recording mediums and rewritable optical recording mediums. The optical head apparatus using a polarization optical system is also as an optical head apparatus adapted to read-only optical recording mediums.
Write-once-read-many optical recording mediums and rewritable optical recording mediums are usually provided with grooves for achieving tracking. When a track error signal is detected from these optical recording mediums, the detection is usually achieved by a push-pull method. The push-pull method involves receiving a reflected light from an optical recording medium by a photo-detector with the reflected light divided into two regions defined by a straight line which crosses the beam axis on a face vertical to the beam axis and extends in the direction corresponding to the tangential direction of the optical recording medium. Assuming that output signals from the photo-detector corresponding to these two regions are referred to as Ia are Ib, the sum signal is given by Ia+Ib, and the push-pull signal is given by Ia−Ib. The track error signal obtained by the push-pull method is given by (Ia−Ib)/(Ia+Ib). One index that represents the quality of the track error signal obtained by the push-pull method is the push-pull signal modulation factor. The push-pull signal modulation factor is obtained by dividing the amplitude of the push-pull signal by the level of the sum signal for the case when the focused spot on the optical recording medium crosses the groove of the optical recording medium. In other words, the push-pull signal modulation factor corresponds to the amplitude of the track error signal obtained by the push-pull method.
On the other hand, read-only optical recording mediums are usually provided with pits for achieving tracking. When a track error signal is detected from this optical recording medium, the detection is usually achieved by a DPD (Differential Phase Detection) technique. The DPD method involves receiving a reflected light from an optical recording medium by a photo-detector with the reflected light divided into four regions defined by straight lines which each cross the beam axis on a face vertical to the beam axis, one extending in the direction corresponding to the radial direction of the optical recording medium and the other extending in the direction corresponding to the tangential direction. Assuming that output signals from the optical detector corresponding to two regions positioned at one set of opposite angles out of the four regions are referred to as Ia and Ic, and output signals from the optical detector corresponding to other two regions positioned at the other set of opposite angles are referred to as Ib and Id, the track error signal (DPD signal) obtained by the DPD method is given by the temporal difference between (Ia+Ic) and (Ib+Id). One index which represents the quality of the track error signal obtained by the DPD method is the DPD signal amplitude. The DPD signal is defined as the amplitude of the DPS signal standardized by the duration of the channel clock for the case when the focused spot on the optical recording medium crosses a pit of the optical recording medium. That is, the DPD signal amplitude corresponds to the amplitude of the track error signal obtained by the DPD method.
In general, the push-pull signal modulation factor and the DPD signal amplitude varies depending on the in-plane position of the optical recording medium. When the push-pull signal modulation factor and the DPD signal amplitude largely vary depending on the in plane position of the optical recording medium, a gain of the track servo which is optimally adjusted for the position where the push-pull signal modulation factor and the DPD signal amplitude are high is excessively low for the position where the push-pull signal modulation factor and the DPD signal amplitude are low, causing the residual error in the track servo. Reversely, a gain of the track servo which is optimally adjusted for the position where the push-pull signal modulation factor and the DPD signal amplitude are low is excessively high for the position where the push-pull signal modulation factor and the DPD signal amplitude are high, resulting in the oscillation of the track servo. Therefore, it is necessary to decrease the changes in the push-pull signal modulation factor and DPD signal amplitude depending on the in-plane position of the optical recording medium, in order to achieve the stable track servo control for all the in-plane positions of the optical recording medium.
The allowed maximum change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium is defined by the written standards of the optical recording medium. The standard for the DVD-R system requires:(PPmax−PPmin)/(PPmax+PPmin)<0.15,where PPmax and PPmin are the maximum and minimum values of the push-pull signal modulation factor, respectively. When the maximum changes in the push-pull signal modulation factor and the DPD signal amplitude are equal to or less than the allowed maximum changes in the push-pull signal modulation factor defined for the DVD-R system, the stable track servo is achieved for all the in-plane positions of the optical recording medium. When the maximum changes in the push-pull signal modulation factor and the DPD signal amplitude exceed the allowed maximum changes in the push-pull signal modulation factor defined for the DVD-R system, this makes it difficult to achieve the stable track servo for all the in-plane positions of the optical recording medium. Therefore, an optical head apparatus and an optical information recording/reproducing apparatus which record or reproduce information onto or from such optical recording mediums require measures for suppressing the changes in the push-pull signal modulation factor and DPD signal amplitude depending on the in-plane position of the optical recording medium.
It should be noted that polycarbonate, which exhibits birefringence, is usually used for the protective layer of optical recording mediums, due to the low cost. The use of an optical head apparatus based on a polarization optical system for recording or reproducing information onto or from an optical recording medium with a protective layer exhibiting birefringence causes reduction in the amount of light received. Here, let us define the relation between the disc 7, which is an optical recording medium, and XYZ-coordinates as shown in FIG. 27. The X-axis is defined as the radius direction of the disc 7, the Y-axis as the tangential direction, and the Z-axis as the normal direction of the disc 7. The protective layer usually exhibits biaxial anisotropy in the refractive index, and the three main axes substantially coincide with the X-axis, the Y-axis and the Z-axis. Assuming that the three principal indices of refraction associated thereto are defined as nx, ny and nz, respectively, and that the values of in-plane birefringence and vertical birefringence are defined as Δni and Δnv, respectively, then, the value Δni of the in-plane birefringence is defined as:Δni=nx−ny,and the value Δnv of the vertical birefringence is defined as:Δnv=(nx+ny)/2−nz.
The value Δni of the in-plane birefringence depends on the manufacturing conditions of the protective layer and varies in accordance with the in-plane position of the optical recording medium. On the contrary, the value Δnv of the vertical birefringence is substantially uniquely determined in accordance with the material of the protective layer, kept approximately constant, independently of the in-plain position of the optical recording medium. When polycarbonate is used as the protective layer, the value Δni of the in-plane birefringence varies over a range of about ±3×10−5, while the value Δnv of vertical birefringence is approximately constant within a range between about 6×10−4 and about 8×10−4. A technique for separately measuring the in-plane and vertical birefringence of the protective layer as thus described is disclosed in, for example, Japanese Laid Open Patent Application No. JP 2004-163225 A. The inventor of the present invention have found out that the above-described changes in the push-pull signal modulation factor and DPD signal amplitude depending on the in-plane position of the optical recording medium is caused by the changes in the in-plane birefringence depending on the in-plane position of the optical recording medium under the existence of the vertical birefringence. The influence of the birefringence is increased as the wavelength of the light source is reduced. Therefore, the HD DVD-R system and the HD DVD-ROM system, which use a light source with a wavelength of about 405 nm for recording/reproducing, experience larger changes in the push-pull signal modulation factor and the DPD signal amplitude than the DVD-R system and the DVD-ROM system, which uses a light source with a wavelength of about 660 nm for recording/reproducing.
FIG. 28 shows a calculation example of the relation between the value of in-plane birefringence and the sum signal level with the value of vertical birefringence used as a parameter, while FIG. 29 shows a calculation example of the relation between the value of in-plane birefringence and the push-pull signal amplitude with the value of vertical birefringence used as a parameter. Additionally, FIG. 30 shows a calculation example of the relation between the value of in-plane birefringence and the push-pull signal modulation factor with the value of vertical birefringence used as the parameter, which is obtained from the sum signal level shown in FIG. 28 and the push-pull signal amplitude shown in FIG. 29. The calculation conditions are as follows: the wavelength of the light source is 405 nm, the opening number of the objective lens is 0.65, the thickness of the protective layer of the optical recording medium is 0.6 mm, the pitch of the groove is 0.4 μm, and the depth of the groove is 25 nm. These conditions correspond to the conditions for the HD DVD-R system. The vertical axes of FIG. 28 and FIG. 29 are standardized at the sum signal level for the case when the groove is not formed on the optical recording medium. Black circles on the drawings indicate the calculation result for the value of vertical birefringence being 0, and white circles on the drawings indicate the calculation result for the value of vertical birefringence being 7×10−4.
As shown in FIG. 28, the sum signal level shows the maximum value when the value Δni of in-plane birefringence is 0, for both of the cases with or without the existence of vertical birefringence, and the sum signal level decreases as the absolute value of the value Δni of in-plane birefringence is increased. On the other hand, as shown in FIG. 29, the push-pull signal amplitude shows the maximum value when the value Δni of in-plane birefringence is 0, similarly to the sum signal level and the push-pull signal amplitude is decreased as the absolute value of the value Δni of in-plane birefringence is increased, for the case when the vertical birefringence does not exist; however, for the case when the vertical birefringence exists, the push-pull signal amplitude is monotonously decreased as the value Δni of in-plane birefringence is changed from positive to negative values. As a result, as shown in FIG. 30, the push-pull signal modulation factor is constant independently of the value Δni of in-plane birefringence, when the vertical birefringence does not exist, while the push-pull signal modulation factor is monotonously decreased as the value Δni of in-plane birefringence is changed from positive to negative values under the existence of the vertical birefringence.
For the case that the value Δnv of vertical birefringence is 7×10−4, the foregoing (PPmax−PPmin)/(PPmax+PPmin) greatly exceeds 0.15, which is the allowed maximum value defined in the standard for the DVD-R system, when the value Δni of in-plane birefringence varies within a range between +3×10−5 (where the difference between the maximum and minimum values is 6×10−5). In order to eliminate the necessity of taking measures for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium, (PPmax−PPmin)/(PPmax+PPmin) is required to be less than 0.15, which is the allowed maximum value defined by the standard for the DVD-R system, and the variation in the value Δni of in-plane birefringence is required to be reduced within a range of ±1.15×10−5 (the difference between the maximum and minimum values is 2.3×10−5). Reversely, unless the variation in the value Δni of in-plane birefringence is reduced within the range of ±1.15×10−5 (the difference between the maximum and minimum values is 2.3×10−5), a measure is required for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium.
For the case that the value Δnv of vertical birefringence is 6×10−4, a similar calculation has shown that, in order to eliminate the necessity of taking measures for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium, (PPmax−PPpin)/(PPmax+PPmin) is required to be less than 0.15, and the variation in the value Δni of in-plane birefringence is required to be reduced within a range of ±1.35×10−5 (the difference between the maximum and minimum values is 2.7×10−5). Reversely, unless the variation in the value Δni of in-plane birefringence is reduced within the range of ±1.35×10−5 (the difference between the maximum and minimum values is 2.7×10−5), a measure is required for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium.
Furthermore, for the case that the value Δnv of vertical birefringence is 8×10−4, a similar calculation has shown that, in order to eliminate the necessity of taking measures for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium, (PPmax−PPmin)/(PPmax+PPmin) is required to be less than 0.15, and the variation in the value Δni of in-plane birefringence is required to be reduced within a range of 1.0×10−5 (the difference between the maximum and minimum values is 2.0×10−5). Reversely, unless the variation in the value Δni of in-plane birefringence is reduced within the range of ±1.0×10−5 (the difference between the maximum and minimum values is 2.0×10−5), a measure is required for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium.
In summary, in order to eliminate the necessity of the measure for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium, (PPmax−PPmin)/(PPmax+PPmin) is required to be less than 0.15 for the value Δnv of vertical birefringence ranging between 6×10−4 and 8×10−4, and the product of Δnv and (Δnimax−Δnimin) is required to be equal to or less than 1.6×10−8, where the maximum and minimum values of in-plane birefringence are defined as Δnimax, Δnimin, respectively. Reversely, unless the product of Δnv and (Δnimax−Δnimin) is reduced to be equal to or less than 1.6×10−8, a measure is required for suppressing the change in the push-pull signal modulation factor depending on the in-plane position of the optical recording medium.
When the value Δni of the in-plane birefringence ranges in a range of ±3×10−5 (the difference between the maximum and minimum values is 6×10−5), on the other hand, a similar calculation has shown that the value Δnv of vertical birefringence should be adjusted to 2.7×10−4 or less in order to reduce (PPmax−PPmin)/(PPmax+PPmin) below 0.15. In other words, (PPmax−PPmin)/(PPmax+PPmin) is reduced below 0.15 by the reduction of the effective value Δnv of vertical birefringence down to 2.7×10−4 or less, which is a measure for suppressing the change in the push-pull signal modulation factor depending on the in plane position of the optical recording medium, even when the value Δni of in-plane birefringence varies over a range of +3×10−5 (the difference between the maximum and minimum values is 6×10−5).
FIG. 31 shows a calculation example of the relation between the value of in-plane birefringence and the DPD signal amplitude with the value of vertical birefringence used as a parameter. The calculation conditions are as follows: the wavelength of the light source is 405 nm, the opening number of the objective lens is 0.65, the thickness of the protective layer of the optical recording medium is 0.6 mm, the pitch of the pits is 0.4 μm, and the depth of the pits is 25 nm. These conditions correspond to the conditions for the HD DVD-ROM system. The vertical axis of FIG. 31 is standardized by the duration of the channel clock. The black circles on the drawing indicate the calculation result for the value of vertical birefringence being 0, and the white circles on the drawing indicate the calculation result for the value of vertical birefringence being 7×10−4.
As shown in FIG. 31, the DPD signal amplitude is constant independently of the value Δni of in-plane birefringence when no vertical birefringence exists; however, under the existence of vertical birefringence, the DPD signal amplitude is monotonously decreased as the value Δni of in-plane birefringence is changed from positive to negative values.
Consideration is then given to the mechanism in which the variation in the in-plane birefringence under the existence of the vertical birefringence causes the changes in the push-pull signal modulation factor and the DPD signal amplitude as mentioned above. When the protective layer of the disc 7 is free from birefringence, the reflected light from the disc 7 is converted into a S-polarized light with respect to the polarizing beam splitter 3 after passing through the quarter-wave plate 4. Therefore, nearly 100% of this light is reflected by the polarizing beam splitter 3 and received by the photo-detector 10. On the contrary, the birefringence of the protective layer of the disc 7 causes the reflected light from the disc 7 to be converted into an elliptically-polarized light after passing through the quarter-wave plate 4. That is, the S-polarized component for the polarizing beam splitter 3 is decreased, while the P-polarized component is generated. Accordingly, nearly 100% of the S-polarized component is reflected by the polarizing beam splitter 3 and received by the photo-detector 10, while nearly 100% of the P-polarized component passes through the polarizing beam splitter 3 and returns to the semiconductor laser 1. This is the reason of the decrease in the amount of light received by the photo-detector 10.
Although the in-plane and vertical birefringences both decrease the amount of light received by the photo-detector, the influences on the light passing through the protective layer of the optical recording medium are different between the in-plane and vertical birefringences. The birefringence of the protective layer offers an optical phase difference between the polarization components in the X-axis and Y-axis directions for the light passing through the protective layer. In the following, the optical phase difference is defined as positive when the phase of the polarization component in the X-axis direction leads ahead of the phase of the polarization component in the Y-axis direction, while the optical phase difference is defined as negative when the phase of the polarization component in the X-axis direction lags behind the phase of the polarization component in the Y-axis direction. The influence of the in-plane birefringence on the light passing through the protective layer of the optical recording medium does not depend on the input direction to the optical recording medium and the incident angle thereof. Positive in-plane birefringence causes a constant negative optical phase difference over a section vertical to the beam axis of the light passing through the protective layer, while negative in-plane birefringence causes a constant positive optical phase difference over the section vertical to the beam axis of the light passing through the protective layer. On the contrary, the influence of the vertical birefringence on the light passing through the protective layer of the optical recording medium depends on the incident direction and angle to the optical recording medium. Assuming that the original point is defined as the intersection of the beam axis on a section vertical to the beam axis of the light passing through the protective layer, a positive optical phase difference is generated in the vicinity of the X-axis in the region of X<0 and the region of X>0 except the region of X=0, while the negative optical phase difference is generated in the vicinity of the Y-axis in the region of Y<0 and the region of Y>0 except the region of Y=0. Also, the optical phase difference generated at the original point is 0, and the absolute value of the generated optical phase difference is increased with the distance the original point.
When the focused spot formed on the optical recording medium crosses the groove or pit on the optical recording medium, the change in the magnitude of the reflected light from the optical recording medium is mainly generated in the region of X<0 and the region X>0 except the region of X=0, in the vicinity of the X-axis. That is, the lights in these regions contribute to the increases in the push-pull signal modulation factor and the DPD signal amplitude. For the case when the in-plane birefringence is positive under the existence of the vertical birefringence, the optical phase differences caused by the in-plane and vertical birefringences are cancelled out in the region of X<0 and the region X>0 except the region of X=0 in the vicinity of the X-axis, thereby decreasing the absolute value of the optical phase difference, while the optical phase differences caused by the in-plane and vertical birefringences are added together in the region of Y<0 and the region of Y>0 except the region of Y=0 in the vicinity of the Y-axis, thereby increasing the absolute value of the optical phase difference. The proportion of light received by the photo-detector is decreased as the absolute value of the optical phase difference is increased, and therefore the proportion of light received by the photo-detector is increased in the region of X<0 and the region X>0 except the region of X=0 in the vicinity of the X-axis, as compared with the region of Y<0 and the region of Y>0 except the region of Y=0 in the vicinity of the Y-axis. This results in the increases in the push-pull signal modulation factor and the DPD signal amplitude. For the case that the in-plane birefringence is negative under the existence of the vertical birefringence, on the other hand, the optical phase differences caused by the in-plane and vertical birefringences are added together in the region of X<0 and the region X>0 except the region of X=0 in the vicinity of the X-axis, thereby increasing the absolute value of the optical phase difference, and the optical phase differences caused by the in-plane and vertical birefringence are cancelled out in the region of Y<0 and the region of Y>0 except the region of Y=0 in the vicinity of the Y-axis, thereby decreasing the absolute value of the optical phase difference. The proportion of the light received by the photo-detector is decreased as the absolute value of the optical phase difference is increased, and therefore the proportion of the light received by the photo-detector is reduced in the region of X<0 and the region X>0 except the region of X=0 as compared with the region of Y<0 and the region of Y>0 except the region of Y=0 in the vicinity of the Y-axis. This results in the decreases in the push-pull signal modulation factor and the DPD signal amplitude.
Substrates, which correspond to the protective layers for substrate-incident optical recording mediums, such as HD-DVDs, are usually manufactured through injection molding. In this case, the in-plane birefringence depends on the position in the radius direction of the optical recording medium, while exhibiting substantially no dependency on the position in the tangential direction. Specifically, the in-plane birefringence is positive in the inner portion of the optical recording medium, and the in-plane birefringence monotonously decreases from the inside to the outside; the in-plane birefringence is negative in the outer portion. Therefore, the push-pull signal modulation factor and the DPD signal amplitude are high in the inner portion of the optical recording medium, monotonously decreasing as it goes from inner to outer portion; the push-pull signal modulation factor and the DPD signal amplitude are low in the outer portion. On the contrary, covers, which correspond to the protective layers for cover-incident optical recording mediums such as BDs, are usually manufactured by punching of sheets. In this case, the in-plane birefringence depends on the position in the tangential direction of the optical recording medium, while exhibiting substantially no dependency on the position in the radius direction. Specifically, the in-plane birefringence alternately shows a positive local maximum value and a negative local minimum value twice at intervals of 90 degrees for one circulation of the optical recording medium. Therefore, the push-pull signal modulation factor and the DPD signal amplitude alternately show a high maximal value and a low minimal value twice at intervals of 90 degrees for one circulation of the optical recording medium.
In relation to the foregoing descriptions, a measuring method of birefringence properties of the optical recording medium is disclosed in Japanese Laid Open Patent Application No. JP-A 2004-163225. In this conventional measuring method of birefringence properties, light is emitted onto a measurement target medium through an objective lens having a numerical aperture equal to or greater than a predetermined numerical aperture, and the light amount of the polarization component in a specific direction within the reflected light reflected on the reflection surface of the measurement target medium is measured and defined as a first amount of light APH. Light is emitted onto the measurement target medium through the objective lens having the numerical aperture equal to or greater than the predetermined numerical aperture, and the light amount of the polarization components in the specific direction and in the direction orthogonal to the specific direction within the reflected light reflected on the reflection surface of the measurement target medium is measured and defined as a second light quantity ANH. The vertical birefringence property of the measurement target medium is determined on the basis of the ratio APH/ANH of the first and second light amounts and the in-plane birefringence property of the measurement target medium.
An optical disc apparatus is also disclosed in Japanese Laid Open Patent Application No. JP-A2003-248118. Formed on a wavelength plate in this conventional optical disc apparatus are micro cyclic structures sized half or less of the wavelength of the light within a targeted wavelength range. Two substrates are prepared which are opposed with each other with the micro cyclic structures shifted by half a cycle and engaged with each other. The interval between these substrates is adjusted to vary the overlap amount of the micro cyclic structure. The thus-configured wavelength plate, which has a combined birefringence structure incorporating birefringence structures each having superior controllability of the birefringence property through changing the shape to provide a variable phase difference, covers a wide usage range, adaptably changing the polarization state for the light of wavelengths over the entire desired wavelength range.
Also, an optical pickup apparatus is disclosed in Japanese Laid Open Patent Application No. JP-A 2004-39018. This conventional optical pickup apparatus emits light onto the recording surface of an information recording medium and receives the reflected light from the recording surface. The optical pickup apparatus is provided with: an optical system including at least one light source, an objective lens for focusing the light beam emitted from the light source onto the recording surface, and an optical element arranged in the optical path of the light beam emitted from the light source and to the objective lens, which element includes an electro-optic crystal having a refractive index distribution in accordance with the voltage applied through an electrode and exhibiting a longitudinal electro-optic effect for compensating the astigmatism component in the wave aberration of the light beam focused on the recording surface, which system guides the return light beam reflected on the recording surface to a predetermined light receiving position; and a photo-detector arranged on the light reception position.